1. Field of the Invention
The present disclosure relates to a battery voltage monitoring apparatus including an RC filter circuit.
2. Description of Related Art
There is known a battery voltage monitoring apparatus capable of detecting a cell voltage of each of battery cells constituting a battery pack, as described, for example, in Japanese Patent Application Laid-open No. 2007-10580. The battery voltage monitoring apparatus described in this patent document is connected with positive and negative terminals of each battery cell to detect the cell voltage of each battery cell.
Generally, as shown in FIG. 6, such a battery voltage monitoring apparatus is provided with a filter circuit as a noise countermeasure. The battery voltage monitoring apparatus shown in FIG. 6 includes a filter circuit 130 and a battery voltage measuring apparatus 120. The filter circuit 130 is disposed between the positive and negative electrodes of respective battery cells 110 constituting a battery pack 100 and the battery voltage measuring apparatus 120.
A wire is connected between each of the positive and negative electrodes of each battery cell 110 and the battery voltage measuring apparatus 120 through the filter circuit 130. For each adjacent two of the battery cells 110, the wire connected to the negative electrode of one battery cell 110 is also used as the wire connected to the positive electrode of the other battery cell 110 except the battery cell 110 on the highest voltage side and the battery cell 110 on the lowest voltage side.
The filter circuit 130 includes resistors 140 respectively interposed in the wires connected between the electrodes of the respective battery cells 110 and input terminals of the battery voltage measuring apparatus 120, and capacitors 150 each connected across adjacent two of the input terminals. One of the resistors 140 and a corresponding one of the capacitors 150 constitute an RC filter as a low-pass filter for each one of the battery cells 110.
When a current pathway across n (n being a positive integer) neighboring battery cells 110 is referred to as “pathway n”, since the pathway n is constituted of two resistors 140 and n series-connected capacitors 150, the transfer function Gain of the pathway n is given by the expression of Gain=1/{1+2 πf·(2R)·(C/n)}, where R is a resistance of the resistor 140, C is a capacitance of the capacitor 150, and f is a cut-off frequency of the pathway n. In this expression, when (2R)−(C/n)=Tn, since Tn is proportional to (1/n), and fn=(1/Tn), the cut-off frequency fn is in proportion to n.
FIG. 7 is a diagram showing variation of the cut-off frequency fn for various values of n. In FIG. 7, f1 indicates the cut-off frequency of pathway 1, f2 indicates the cut-off frequency of pathway 2, f3 indicates the cut-off frequency of pathway 3, and f12 indicates the cut-off frequency of pathway 12. As seen from FIG. 7, the cut-off frequency increases with the increase of n, that is, with the increase of the number of the battery cells 110 or the capacitors 150 connected in series. For example, when the battery pack 100 is constituted of twelve battery cells 110 as shown in FIG. 6, the maximum cut-off frequency is twelve times as high as the minimum cut-off frequency. Hence, the conventional battery voltage monitoring apparatus as described above has a problem in that the different pathways have different cut-off frequencies.